The use of acoustic energy to levitate or position an object within a chamber has many uses, especially in satellites to hold objects under almost zero gravity without touching them with a solid support (containerless processing). In acoustic positioning, one or more transducers apply sonic energy to the chamber at one or more frequencies that are resonant to the chamber and that force the object towards a stable equilibrium position lying away from the walls of the chamber. A positioned object often undergoes translational motion, rotational motion, and/or drop shape oscillation (oscillation in shape) which may be accidental or which may be purposely caused. U.S. Pat. No. 4,773,266 by Barmatz et al describes techniques for inducing or damping motion and describes several useful types of information that can be gleaned from knowledge about motion of an acoustically positioned object. However, measurement of object motion can be difficult.
Where the object is positioned at room temperature in a chamber with transparent walls, object motion can be observed directly or detected by light beams directed across the path of the object onto photo cells. However, in many cases such as where the object is processed at high temperatures, it is very difficult to directly observe the object or measure its motion by light beams. A simple technique which allowed measurement of object motion in translation, rotation, and/or drop oscillation motion, would have many uses.